Summer 2018 and LRFDdesign about ASD know wanted to knew you you never Everything appropriate resistance factor from always things inplace such asheadblocks inatheatre’sgrid fly Design) appropriate factor safety structures. useto engineers ensuremethods adequately design safe they andR Load philosophies design The community regarding shouldbe. just how margin large thissafety safety. of margin service. In fact, you wantto have thatprovides capacity a extra overstressedbe failure by put on normal itandrisk theloads during structure tostructure support. Pretty simple. the designs member theengineer must lessthanwhat be structural more.knowing gothen into I’ll greater depth for thosereaders are who interested in thedifferencesbroad of overview between thetwo approaches and (Ihope)enlighten you. will this article a I’m with goingoff to start acronyms For anyone whohasencountered design theengineering Design strength :RFD nominal L Design strength multiplied by the loads along astructure/member with : of Load Dead Self-weight ASD – Allowable Strength: Before going further, here’s for easyreference: glossary a short There engineering isgeneralconsensus inthestructural In structural engineering,thedemands placed or on In astructure structural A ASD andLRFDwondered are, theheck they what llowable (formerly Strength Design esistance Factor (LRFD)are Design thetwo recognized ASD nominal strength by divided the A llowable (ASD)and Strength Design Y ou don’t to wantthestructure A llowable Stress Summer 2018 By

Miriam Paschetto 24 of thestructure’sof strength. yield from on effects thestructure The the isthe This “allowable” strength of the structure’s loads. utilized canbe under applied service capacity factored andthestrength isultimate thanyield. rather R Load factored strength in yield are loads when removed.) shape return to itsoriginal experiences deformationstructure (thatis, permanent itdoes not to yield. material starts bendingjust atthepointwhereassociated theouter with surface the demands put by on thestructure theloads, either bendingatthepointwhere theentire with cross isyielding section or analyzed member designed being structural thatareloads lesspredictable thandead loads SD provides the desired factor of safety by ASD provides how limiting safety thedesired much factor of of In thebroadest strokes, usesunfactored theengineer and loads Phi – Omega Yield ator just below strength capacity –The thepointwhere the amember modulus, of Section property S–Geometric to needed or exceed meet Required capacity strength –Structural psi –Pounds square per inch plf –Pounds linealfoot per amember associated modulus, of Plastic property Z–Geometric Nominal strength –Unfactored or thestructure of capacity Live –Most load often from loads but people more generally, andR LRFD –Load –LRFDresistance factor esistance andstrength Factor loads are (LRFD)both Design SD safety factor for bendingmomentASD safety esistance Factor Design A llowable in (ASD)while Strength Design SD and it is in the range of 60% 60% of ASD anditisintherange structural designs. different routes tosafe ASD andLRFD, whichprovide the sameriverare muchlike Different bridgescrossing ASD or LRFD Summer 2018 FD. Let’s look Let’s LRFD. ASD or 1.2x 5 plf = 6 plf 0.6 x 250 lb = 400 lb °  °  Design loads – use 1.2 dead load factor and 1.6 live load Design loads load – use 1.2 dead and 1.6 live factor factor: M = 1,075'-lb bending moment, strength, Required Phi = 0.9 factor, Resistance = 31,500 psi x Fy Fb = phi_b Design stress, = M/Fb = required Z, plastic section modulus, Minimum 0.41 in^3 S than minimum greater 1-½ STD pipe = 0.421 in^3, has Z OK required, Design loads – as shown M = 687.5'-lb bending moment, strength, Required Omega = 1.67 Safety factor, = M/Fb = 0.394 required S, section modulus, Minimum in^3 S less than minimum 1-½ STD pipe = 0.309 in^3, has S try pipe no good, larger size required, S than minimum greater 2 STD pipe 0.528 in^3, has S = OK required, Allowable stress, Fb = Fy/Omega = 20,958 psi stress, Allowable Yield stress, Fy = 35,000 psi Fy stress, Yield Yield stress, Fy = 35,000 psi Fy stress, Yield LRFD ASD •  •  • •  •  •  •  •  •  • •  • •  •  •  n n specially when designing members to resist bending moment, bending moment, Especially resist when designing to members method which is being employed it is important know to Clearly, FD will often mean a more efficient design as in this example. design example. efficient in this as LRFD will mean a more often the necessary be a 2" STD. to was determined pipe size ASD, With The designer is permitted of a pipe size LRFD design 1 ½" STD. gave in LRFD. strength inherent of more utilize a member’s to that vigorously people state Many process. design analysis or in any I both be the same project. used on ASD and LRFD should never will further that address because on in reality it sometimes makes track of careful keep to taken are as steps as long so, do to sense important always to grasp just that it’s now, For being done. what’s are loads and strengths whether any know a very lead to would one up: get things mixed to ways at the two than necessary) stronger structure and the other (much conservative be described the structure could as being where on a situation to ice.” “thin 25 PROTOCOL ote ote ith W ASD loads a capacity demand that is ASD. esistance Factor Design, on the other hand, hand, the other on Design, Factor esistance SD the loads are used “as is” and a structure’s and a structure’s is” “as used ASD the loads are SD. Thus, even though all it might even the look like Thus, ASD.

5 plf dead load – estimate of pipe batten’s self-weight plus 5 plf load dead of – estimate pipe batten’s weight scrim’s piece scenic load – point load from 250 lb live Span = 10' (supported at each end) Span = 10' (supported at each Loads: •  •  n n

FD, Load and R LRFD, To review, in review, To a little bit strengths ultimate the elastic versus I am glossing over illustrate with a very of simple example sizing a pipe batten: To Thus, the necessary the safety margin designingThus, is maintained by achieves a safe structural design by a slightly different route. structural a safe route. achieves a slightly design different by FD, both the loads and the nominal strength are factored to to factored are both the loads strength and the nominal LRFD, the on demands The anticipated the designdetermine parameters. the than 1.0 to applying greater factors by increased structure are its reducing by is calculated design strength The structure’s loads. N a specified by percentage. strength ultimate or LRFD nominal to “allowable” as to referred is never that an LRFD design strength with confusion prevent by reduced strength, its elastic failure to is equal design strength at the same time and increased LRFD the loads are In factors. to is slightly reduced at failure strength ultimate the structure’s design used for loads are Increased the designdetermine strength. the to closer can be the design taken strength in LRFD and therefore point. failure material’s the cases LRFD allows that in many to know is sufficient It here. capacity than of advantage of take more designer to the structure’s is permitted in the same thing in methods amount to in the two factors different That is, design. efficient in a more LRFD will result often the end, LRFD will being used. a lighter member sometimes lead to applied unfactored or service loads are calculated and the required service or and the required calculated unfactored applied loads are strength. the allowable to these effects is compared resist to strength yield enough large so that the resulting strengths have structures to to required the strength to equal than or is larger strength allowable support If the service the loads. the structure then needs another strength, than the allowable greater as is. the structure adequate is of Ifround not, beef design to it up.

under applied service loads. under applied service capacity can be utilized capacity can be utilized how much of the structure’s how much of the structure’s factor of safety by limiting factor of safety ASD provides the desired ASD provides Summer 2018 category of non-linear structures are non-linear structures tensile roofs. fabric of category anticipated, unfactored asare loads in used three. Therefore, for to theactual, needs designed thestructure be changesecond by andthestress time another will multiple, maybe There isnoeasily discernible pattern; could theloads doubled be a not double. It changes by some other multiple, could 1.8or be 2.5. example, are theloads doubled thestress does inthestructure if response does notchange atthesame rate astheapplied loads. For non-linear structure. isnon-linear thestructure’s Something when rough estimate. a to with determine feasibility isonlymoment trying theengineer in their head. useful inmany canbe This situations where atthe determine amember size by doing thenecessary to usethanLRFD; canfrequently engineer anexperienced design many use still engineers supports the latest still Manual Steel of Construction to started movethe steel inthesamedirection, industry although reinforced LRFDfor designing adoption of concrete. By the1990s, that would eventually inthe1970sto lead theconcrete industry’s and the always wasbecoming shape) returnswellunderstood to itsoriginal behavior (thatis, andit you when astructure andunload load developing on based theresults. theories By 1900s, theearly elastic and experiments running began Then inthe1800sengineers through experience. analysis. There wasnosuch thingasstructural thumb members weredeveloped structural of sized on based rules many such things, historical. theanswer ispartly ago, Centuries strengths, “Is this constantly loads, given askwhen will or member of results, aset or at itsfailure strength. more anditwouldillustration, on thestructure load be only 11psf community, that’s error. anunacceptably of slimmargin In the 10% away from failure. theengineering on theexperience Based of when loaded that by showgrid is only loads rigging a flying Imagine strength,the LRFDdesign there margin. would safety asmall be above andthen compares theassociated required strength against usingthe110psf designs have safety. agrossly inadequate factor of theengineer Consider if for andthen LRFDstrength, designs structure will thestructure over-designed by roughly 50%, which wastes material andmoney. allowable strength. isnotdangerous This but would thestructure be compared be factored thehigher will to loads therelativelyof low safety. factor of larger thannecessary on effect thestructure The todesigns an More isthat important As asthe1930sand1940s, early theresearch done wasbeing But, why usetwo different approaches design atall? thetwo iswhy dangerousThis methods engineers mixupof If, however, applies unfactored theengineer applies LRFDfactored anengineer to but loads astructure If SD method grew out of thatknowledge. grew out of ASD method ASD allowable strength, have will thestructure a ASD or LRFD?” SD for designing steel.ASD for designing It easier isslightly ASD combined from load theillustration SD is necessary when designing a designing when ASD isnecessary ASD. SD loads toASD loads the ASD calculations A n important n important ASD. Indeed, W ell, aswith Summer 2018 26 for engineers’ “back-of-the-envelope” calculations. alwayswill have aplace—not just for non-linear structures, but also to more efficient structures, saving on materials and cost. But world shouldsimply switch to andanalysis. LRFDfor design It leads the appropriate safety. factor of placed are on thestructure correct canthen apply andthedesigner isexpected tostructure see. way, That be thedemands thatwill to much be lower asisindicated for thelinearbehavior line. published pastarticlesin Engineers andtheStructural Engineers Association ofNew York. Miriamhas Safety Working Group. Sheisalsoamemberofthe American SocietyofCivil She iscurrentlyparticipatinginthe Structures Task GroupoftheEvent and AluminumTrussesand Towersthe TemporaryStructures Task Groups.on active inESTA’s Riggingand Event Safety Working Groupsandhasserved Super Bowlhalftimeshowsandoutdoormusicfestivals’ stages. Miriamis for the150thCincodeMayoCelebration inPuebla, Mexico; andnumerous Structures Congress. 2015 EventSafetySummit, andthe2011Structural EngineeringInstitute’s design. loads. There to asneeded ensure isnotasmuch capacity extra asafe ismuch loads closer tounder service thedemand under thefactored were linear. thestructure would loads much be than if higher under service usingfactoreddesigned loads, then thedemand on thestructure vs demand. thestructure—loads theengineer behavior of If non-linear structures. solidlinerepresents The thenon-linear In summary, engineering itseems thatthestructural perhaps W Below is a graph illustrating why for used illustrating LRFDshouldnotbe Below isagraph ith W ASD, thatthe for loads theactual designs theengineer hen usingLRFD, demands load thedesire isfor theservice A s can be seen,s canbe thedemand on thenon-linear structure for NotreDame’s JoyceCenter; theoutdoorstage for SunTrust Park in Atlanta; riggingcapacity analysis King’s newarena; theoutdoorsoundsystemsupports scoreboard supportstructuresfortheSacramento around theworld, amongthemthecenterhung engineering servicesforentertainmentprojects For thepast16yearsshehasprovidedstructural State andworksasan Associate atGeigerEngineers. Miriam Paschetto isalicensedP. E. in New York Protocol andgivenpresentationsatNAT EAC, the n ASD